Earnings Per Common Share - Schedule of Computation of Basic and Diluted Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Income from continuing operations, net of tax	$ 2,117	$ 3,770	$ 4,012
Income (loss) from discontinued operations, net of tax	(135)	(19)	38
Net income	1,982	3,751	4,050
Dividends and undistributed earnings allocated to participating securities	(13)	(24)	(20)
Preferred stock dividends	(265)	(214)	(158)
Net income available to common stockholders	$ 1,704	$ 3,513	$ 3,872
Total weighted-average basic shares outstanding	484.2	504.9	541.8
Effect of dilutive securities:
Stock options	2.5	2.0	2.6
Other contingently issuable shares	1.2	1.3	1.3
Warrants	0.7	1.6	2.3
Total effect of dilutive securities	4.4	4.9	6.2
Total weighted-average diluted shares outstanding	488.6	509.8	548.0
Basic earnings per common share:
Net income from continuing operations (in dollars per share)	$ 3.80	$ 7.00	$ 7.08
Income (loss) from discontinued operations (in dollars per share)	(0.28)	(0.04)	0.07
Net income per basic common share (in dollars per share)	3.52	6.96	7.15
Diluted earnings per common share:
Net income from continuing operations (in dollars per share)	3.76	6.93	7.00
Income (loss) from discontinued operations (in dollars per share)	(0.27)	(0.04)	0.07
Net income per diluted common share (in dollars per share)	$ 3.49	$ 6.89	$ 7.07